Liquid crystal display apparatuses for image display are widely used as large-sized displays of flat-screen televisions and monitors, etc. The liquid crystal display apparatuses employ a backlight unit that applies light to a liquid crystal display panel from the rear or lower side to enhance the luminance of the display screen.
The backlight unit has a lightguide plate and a light-emitting diode (LED) or other light source disposed to face a part of the peripheral side surface of the lightguide plate. The lightguide plate guides light from the light source through it and emits the guided light toward the liquid crystal display panel from the whole of a light-exiting surface of the lightguide plate that faces the display panel.
Recently, as liquid crystal television systems increase in size, there has been an increased demand for further reduction in weight and thickness of lightguide plates used in them. In this regard, as the thickness of a lightguide plate is reduced, it becomes more likely that color irregularity and luminance unevenness will occur on the light-exiting surface of the lightguide plate. If injection molding is employed as a method of producing a large-sized lightguide plate, it is difficult to fill the resin material throughout the molding tool for the large lightguide plate. It is necessary in order to solve this problem to increase the injection pressure, which, however, will cause an increase in the cost of equipment.
Under these circumstances, there has been proposed a backlight unit comprising a plurality of light-emitting units having respective lightguide plates arranged lengthwise and widthwise and a plurality of LEDs provided for each lightguide plate to emit light into it (see Japanese Patent Application Publication No. 2007-293339). In this backlight unit, the LEDs are driven for each light-emitting unit to perform local dimming. More specifically, the outputs of the light-emitting units constituting the backlight unit are controlled for each light-emitting unit on the basis of image data input to the liquid crystal display panel to control the output (brightness) for each light-emitting unit, thus reducing the power consumption and improving the contrast of the liquid crystal display screen and the moving picture performance, for example.
There has also been proposed a liquid crystal display apparatus having a liquid crystal display panel that displays an image signal input to it and a backlight unit having a plurality of light-emitting units comprising RGB light-emitting diodes, which are arranged lengthwise and widthwise. The image signal is divided into blocks corresponding to the size of each light-emitting unit. The luminance and image data of each block are analyzed, and the luminance of each light-emitting unit is set based on the result of the analysis (see Japanese Patent Application Publication No. 2008-90076). That is, this liquid crystal display apparatus also controls the luminance for each light-emitting unit to perform local dimming for improving the image quality (contrast and color reproduction range) and reducing the power consumption in the same way as the liquid crystal display apparatus disclosed in the above-described Japanese Patent Application Publication No. 2007-293339 and so forth.
The above-described conventional techniques, however, still have the following problems to be solved.
With the conventional techniques, the luminance is controlled for each light-emitting unit; therefore, it is difficult to perform control for a screen area smaller than the light-emitting unit and hence difficult to effect more precise local dimming. In addition, it is likely that there will be a large difference in luminance at the boundary between the light-emitting units.